The basis of new, parallax-free methods of length measurements by optical interferometry is presented. Two new length specifying parameters of a material end standard, i.e. optical and mechanical lengths of a block are introduced, that give the possibility to realize more accurately the length unit. A traceability of the mechanical and optical lengths, obtained by optical interferometry, to the basic physical quantity, the length of a material artifact, has been established for the first time. We present here the system of equations which shows the inter-relation between these lengths, and links them to the results of the basic interferometric length measurements and to such quantities as skin depth, roughness values of the gauging surfaces, wringing in surface deformations, excessive wringing film thickness, interferometer optics distortions. Experimental confirmation of the main theoretical relations is presented. We discuss in detail the key feature of optical and mechanical length measurements and give examples of their uncertainty budget evaluations.